The present invention relates in an aspect to a digital controller that outputs pulse-width modulated (PWM) signals and uses feed-back of the output signal to correct for any errors. It further relates to an implementation where the feedback signal is derived from the output of an analog to digital converter (ADC), to create a ‘mixed-signal PWM controller’.
A primary application of such a controller is an audio amplifier, where the PWM signal can be used to drive a switching (class-D) amplifier. After the switching amplifier there is usually an output filter provided to remove high-frequency switching components and make a smooth output signal. Said output signal may be fed to a speaker. The ADC in such a controller is capable of measuring the signal directly at the speaker, i.e. after the output filter. The digital controller can subsequently be configured further e.g. to have a high loop gain to suppress non-idealities in the signal that may arise in the switching amplifier and the output filter.
Traditionally switching amplifiers used either no feedback at all (FIG. 1a), or they used analog feedback loops with a feedback taken before an output filter (FIG. 1b). These analog systems usually have only moderate filter complexity (most commonly a 2nd order loop-filter). This is found to result in less loop-gain and less suppression of non-idealities from the switching amplifier, and even no suppression at all for non-idealities that originate in the output filter.
It is in principle possible to increase the gain of an analog feedback loop by increasing its filtering order. However, this is not often done, as the variability of analog filter components requires either a complex calibration mechanism, or requires a significant back-off margin to allow for tolerances. A back-off margin mitigates most of the advantage of a higher-order filter.
It is also possible to use analog loop-filters with feedback after the output filter (see FIG. 1c) [Adduci et al. in “Switching Power Audio Amplifiers with High Immunity to the Demodulation Filter Effects”, J. Audio Eng. Soc. Vol. 60, No. 12, December 2012]. However, to keep a loop-filter stable further measures have to be taken typically, such as the output filter has to be compensated. In an analog case compensation of the output filter is usually done with a secondary local feedback loop. This is found to reduce an effectiveness of error suppression of the outer loop (at high frequencies) and in addition requires large passive components that also need to be programmable to track component variations in the output filter.
Compensation of the (LC) output filter could be more efficient in the digital domain. With careful design of a digital compensation filter, such is possible in a single loop with full global feedback (see FIG. 1d) [Mouton et al. in “Digital Control of a PWM Switching Amplifier with Global Feedback”, AES 37th Int. Conf., 2009].
Digital implementation of a loop-filter in combination with feedback after an output filter does require an ADC to digitize the output signal. This ADC preferably has a high-resolution for audio-grade signal conversion in combination with a low latency to avoid degradation of the loop stability. The ADC is preferably also tolerant towards a residue of high-frequency switching components.
A combination of above ADC requirements in full or in part is atypical. The requirements could be met by using either a costly overdesigned general purpose ADC [e.g. Mouton above], or by using an ADC that is specially tailored for the application [WO 2014/094913 A1].
Various patent documents recite digital amplifiers.
A very generic patent is WO 2002/078179 A1, relating to class D amplifiers with digital processing in many varieties.
U.S. Pat. No. 6,498,531 B1 recites a system with analog to digital converters for feedback, both before and after an output filter, being an embodiment of a real digital feedback amplifier. Said system is, with its two feedback loops, not optimized for high loop gain and only has a first-order loop-filter.
WO 2009/153449 A1 and WO 2009/153450 A1 recite Digital-input Class-D Audio Amplifiers with digitized feedback in combination with secondary local feedback loops.
Some examples of prior art programmable pulse width modulators can be found in DE 10 2012 102504 A1, US 2005/052304 A1, and WO 2013/164229 A1, whereas Iftekharuddin et al. in Applied Optics, Optical Soc. America, Washington D.C., Vol. 33, No. 8, Mar. 10, 1994, p. 1457-1462 describes background art relating to a butterfly interconnection network. DE 10 2012 102504 A1 recites a PWM in a data-converter which uses adaptable limiters, but is otherwise considered not very flexible as it can not be adapted nor programmed as a whole, let alone individual components thereof. For instance the loop filters 300 are not programmable, as the coefficients have fixed values. It shows only one PWM having two outputs, which outputs are inherently dependent of one and another. It comprises a multiplexer for selecting inputs, but it is not capable of mixing signals. US 2005/052304 A1 recites a PWM modulation circuitry with multiple paths that are nominally out of phase and are combined in an analog summer. But again, the loop-filter components are not programmable nor can their outputs be mixed. Instead, they perform a dedicated noise-shaping function specific for this data converter. WO 2013/164229 A1 describes a class-D audio amplifier with adjustable analog loop filter, but this adjusting is done automatically between a limited number of pre-defined options, depending on the modulator frequency setting. This is very different from the fully programmable digital multi-purpose loop-filter presented here.
It is an objective of the present invention to overcome disadvantages of the prior art digital audio converter and amplifier controller without jeopardizing functionality and advantages.